Process for concentrating hydrofluoric acid



Aug. 21, 1951 c. B. LINN 2,564,910

PROCESS FOR CONCENTRATING HYDROFLUORIC ACID Filed Feb. 28, 1947 Patented Aug. 21, 1951 PROCESS FOR CONCENTRATING HYDROFLUORIC ACID Cari B. Linn, Riverside, Ill., assigner to Universal Oil Products Company, Chicago, Ill., a corpon ration of Delaware Application February 28, 1947, Serial No. 731,730

(Cl. ,2S-153) 11 Claims.

This invention relates to a method for concentrating hydrouoric acid `fromfaqueous solutions of hydrogen rluoride and particularly from a constant boiling mixture of hydrogen fluoride and Water.

An object of this invention is to produce hydroliuoric acid of higher hydrogen fluoride concentration from an aqueous solution of hydrogen fluoride.

Another object of this invention is to produce hydrofluoric acid of from about 90 to about 100% hydrogen fluoride concentration from the hydrogen uoride-water azeotrope.

One specific embodiment of this invention relates to a process for concentrating hydrouoric acid which comprises contacting an aqueous hydroiluoric acid With a polyluoro-hydrocarbon, separating a solution comprising essentially poly- -uoro-hydrocarbon and dissolved hydrofluoric acid from an aqueous solution containing less hydrogen fluoride than the hydrouoric acid charged to the process, separating the first named solution into hydrouoric acid and recovered polyiiuoro-hydrocarbon, recovering said hydrofluoric acid, and recycling the recovered polyiluoro-hydrocarbon to contact with the aqueous hydrofluoric acid charged to the process.

Another embodiment of this invention relates to a process for concentrating hydrouoric acid which comprises contacting an aqueous hydroiiuoric acid with a lluorocarbon, separating a solution comprising essentially a iluorocarbon and dissolved hydrofluoric acid from an aqueous solution containing less hydrogen iiuoride than the hydroiiuoric acid charged to the process, separating the iirst named solution into hydrofluoric acid and recovered iiuorocarbon, recovering said hydrofluoric acid, and recycling the recovered uorocarbon to contact with the aqueous hydrofiuoric acid charged to the process.

A further embodiment of this invention relates to a process for concentrating hydrofluoric acid which comprises countercurrently contacting an aqueous hydrofiuoric acid and a fluorocarbon, separating a solution comprising essentially fluorocarbon and dissolved hydroiluoric acid from an aqueous solution containing less hydrogen fluoride than the hydrouoric acid charged to the process, separating the lirst named solution into hydroiiuoric acid and recovered fluorocarbon, recovering said hydrouoric acid and recycling the recovered fluorocarbon to contact with the aqueous hydrofluoric acid charged to the process.

The process for concentrating hydrogen -fluoride is effected by taking advantage of the im- 2 miscibility of a liquid uorocarbon with water and with aqueous solutions of hydrogen uoride and the relatively higher solubility of hydrogen uoride in said fluorocarbon.

Simple fractional distillation can be used to produce anhydrous hydrogen fluoride from hydrofluoric acid of a hydrogen fluoride concentration higher than that of the constant boiling mixture with water which contains about 38% by weight of hydrogen fluoride, but heretofore no simple method has been available for concentrating hydrogen fluoride from its constant boiling aqueous solution which has a boiling point of 111 C. at a pressure of one atmosphere. By my process substantial enrichment of hydrogen uoride relative to water is obtained by extracting an aqueous solution of hydrogen uoride with a fluorocarbon. The fluorocarbon employed in this process is preferably a liquid at a temperature of 20 C. and at a pressure of one atmosphere absolute but other luorocarbons with lower boiling points are utilizable by carrying out the process under a super-atmospheric pressure. The removal of hydrogen fluoride from an aqueous hydrouoric acid solution is carried out by extracting the aqueous solution with a iluorocarbon and recovering from the fluorocarbon the extracted hydrogen fluoride. This process may be carried out as a liquid-liquid extraction at a temperature as low as the freezingl point of the aqueous hydrogen fluoride solution that is extracted by the uorocarbon. Depending upon the exact temperature and nuorocarbon used it may be necessary to carry out the extraction at a superatmospheric pressure in order to maintain the luorocarbon in substantially liquid phase. It is preferable to carry out the extraction at the autogenous pressure. developed by the fluorocarbon and hydrogen fluoride present in the hydrogen fluoride concentrating system, although higher pressures mayy also be used. Constant boiling hydrofluoric acid freezes at about 35 C. while some of the luorocarbons are liquid at even lower temperatures although those of higher molecular weights are solid at room temperature. The freezing points of hydrogen nuoride-water mixtures other than the constant boiling mixture may be obtained from the approximately straight line curve obtained by plotting the composition versus freezing point based upon the above indicated values and the freezing point of pure Water.

Fluorocarbons which are utilizable in my process comprise the substantially saturated urocarbons which include completely fluorinated paraffins and cycloparafns sometimes also referred to as perfluoroparaims and perfluorocycloparaffins. A series of iluorocarbons corresponding to the parafnic hydrocarbons has the general formula CnF2n+2 which is analogous to the general formula CnH2n+2 of the parainic hydrocarbons. Of the completely fluorinated paraiiins, those having from 3 to 12 carbon atoms per molecule are preferred solvents for extracting hydrogen fluoride from its aqueous solutions. In addition to the fluorocarbons which contain only carbon and fluorine, there are other polyiluorinated hydrocarbons and polyfluorochlorohydrocarbons, such as triuorobenzene or dichlorohexauoropropane which are suitable for concentrating hydrogen fluoride from its aqueous solutions, either the constant boiling solution or one containing more or less hydrogen fluoride than present in its constant boiling aqueous solution.

A few of the physical properties of some of the completely fluorinated parafns are given in the following table:

Other saturated iiuorocarbons suitable for use in my process comprise completely fluorinated cycloparaiiins, such as completely fluorinated cyclopentane, cyclohexane, dimethylcyclohexane, etc. Completely iluorinated cyclopentane which is also referred to as decafluorocyclopentane has a melting point of about 12 C., a boiling point of 23 C., and a density of 1.72 at 02 C., 1.69 at 8.8" C., and 1.67 at 20.8 C. Similarly completely iiuorinated cyclohexane, CSFM, also referred to as decafluorocyclohexane boils at 51 C. and has Ji a density of 1.65 at 0.2o C., 1.63 at 8.8Q C., and 1.60 at 20.8 C. Also completely fluorinated dimethylcyclohexane which is sometimes referred to as peruorodimethylcyclohexane has the general formula CsFie. These different completely vfluorinated paraiiins and cycloparaiiins which may also be referred to as saturated uorocarbons have boiling points very near to or slightly higher or lower than those of the hydrocarbons with corresponding carbon structures.

The use of my method makes it possible to recover hydrofluoric acid of higher hydrogen fluoride concentration from aqueous solutions and sludges that previously could not be concentrated economically and that contained sufficient hyii.

drogen fluoride to present serious disposal problems. These dilute solutions and sludges could be neutralized with caustic soda or by other means but the resultant sodium fluoride is also diflicult to dispose of because of hazards to public health, fish, game, etc., if these wastes are dumped where they may reach rivers, lakes, and other sources of water supplies.

My method for concentrating hydrouoric acid from aqueous solutions of hydrogen fluoride is illustrated in the attached diagrammatic drawing which shows one method by which the process may be carried out.

An aqueous solution of hydrogen fluoride such as constant boiling hydrouoric acid is introi orocarbon solvent.

duced through line l and valve 2 to pump 3 which discharges through line d and valve 5 into the lower part of absorber 5 which may comprise a vertically disposed cylindrical vessel provided with baliles, screens, or other suitable mixing devices. The aqueous hydroiluoric acid is then passed upwardly through absorber 6 wherein it is contacted with a iiuorocarbon introduced from an outside 'source through line 'I and valve 8 and also recycled from line i0 as hereinafter set forth to distributor 9 located in the upper section of absorber 8. In absorber ii, the iiuorocarbon, which is immiscible with the aqueous solution and in which hydrogen fluoride is more soluble than in water, is introduced near the top of said absorber and passes downwardly therethrough couutercurrent to the aqueous hydrogen fluoride introduced near the bottom of absorber S. The aqueous solution of hydrofluoric acid from which a substantial proportion of the hydrogen fluoride is removed by the descending iluorocarbon is discharged from the top of absorber 5, through line I0 and valve Il to waste or to other use or recovery treatment not indicated in the diagrammatic drawing.

The iiuorocarbon containing dissolved hydrogen fluoride is withdrawn from the bottom of absorber 6 through line l2 and valve I3 to separator M which comprises a settling tank or other suitable equipment in which the solution of hydrogen fluoride in the iiuorocarbon is permitted to settle and to separate from small amounts of aqueous hydrogen fluoride solution that may be admixed therewith or entrained therein when removed from the bottom of absorber 6. The aqueous solution of hydrogen fluoride being lighter than and immiscible with the iluorocarbon layer containing dissolved hydrogen fluoride is withdrawn from separator ld through line l5 and valve lli to waste or to other treatment not indicated in the diagrammatic drawing.

From the bottom of separator la, the fluorocarbon containing dissolved hydrogen uoride is withdrawn through line il and valve i8 by pump le which discharges through line 2e and valve 2i into fractionator 22 provided with reboiler coil 23. In fractionator 22 the fluorocarbon containing dissolved hydrogen fluoride is subjected to distillation or desorption treatment to separate dissolved hydrogen iiuoride from the flu- Hydrogen iluoride vapors evolved in fractionator '22 are directed therefrom through line 24 and valve 25 to condenser 2'6 and from thence the mixture of liquid and vapor is directed through rundown line 2l and valve 28 to receiver E@ provided with gas release line 30 containing valve 3| and with liquid drawof line 32 containing valve 33. From receiver 29, the hydrofluoric acid so concentrated by the process of this invention may be withdrawn and utilized as catalyst for hydrocarbon conversion reactions or for other purposes.

Sometimes the material collected in receiver 29 may also contain a certain amount of fluorocarbon. In such cases two liquid layers will be present in receiver 2S, that is, an upper layer consisting of hydrogen fluoride or a hydrogen fluoride-water mixture more concentrated than that charged to absorber 6 and a lower fluorocarbon layer, the latter being suitable for recycling to absorber 6 by means not indicated in the drawing.

The fluorocarbon solvent from which hydrogen fluoride is removed in fractionator 22 is withdrawn from the bottom of fractionator 22 through line 34 and valve 35 to cooler 36 from which the than that employed in absorber 6. For example,

absorber t may be operated at a temperature of from about to about 20 C. and at a pressure of from about 2 to about 10` atmospheres to effeet absorption in a fluorocarbon of hydroiluoric acid containing 90 to 100% hydrogen fluoride and desorption of the dissolved hydrofluoric acid is effected in fractionator 22 also at a temperature of from about 10 to about 20 C. but at an absolute pressure of about 0.5 atmosphere or less. When operating in this manner, the `fluorocarbon from which absorber hydrouoric acid has been removed need not be cooled before being recycled to absorber 6 as hereinabove set forth.

My process for concentrating hydrofluoric acid from its aqueous solutions such as constant boiling aqueous hydrouoric acid or hydrouo-ric acid containing a smaller concentration of hydrogen fluoride than that present in the constant boiling mixture may thus be` carried out in a continuous manner using a fluorocarbon, a mixture of fiuorocarbons, poly-fluorinated hydrocarbons, a poly fluoro-poly chlorohydrocarbon or a chlo- 'rofluorocarbon as solvent for extracting hydrofluoric acid of high hydrogen fluoride concentration from aqueous hydrogen fluoride.

The process of this invention is illustrated further by the following example which should not be misconstrued to limit unduly the broad scope of the invention.

One volume of a liquid perfluoroheptane, CvFie, is contacted thoroughly with one volume of aqueous hydrofluoric acid containing 36% by to fractional distillation removing sufficient Water to form a residue of constant boiling aqueous hydrogen fluoride which is subjected to further treatment in the process to extract hydrogen fluoride therefrom.

The fluorocarbon layer which is withdrawn from the settling chamber is distilled and essentially pure hydrogen fluoride is distilled overhead and collected. The perfluoroheptane from which the dissolved hydrogen fluoride is removed by distillation is then contacted with an additional quantity of aqueous hydrofluoric acid containing 36% by weight of hydrogen fluoride to effect the concentration of a further quantity of hydroiiuoric acid.

The nature of the present invention is evident from the preceding specication and example, although neither section should be misconstrued to limit unduly the broad scope of the invention.

I claim as my invention:

1. A Vprocess for concentrating. hydrofloic acid which comprises contacting an aqueous hydrofluoric acid with a polyuoro-hydrocarbon, separating a solution comprising essentially polyiluoro-hydrocarbon and dissolved hydrouoric acid from an aqueous solution containing less hydrogen fluoride than the hydrofluoric acid charged to the process,separating the first named solution into hydrofiuoric acid and recovered polyiluoro-hydrocarbon, recovering said hydroiiuoric acid, and recycling the recovered polynuoro-hydrocarbon to contact with the aqueous hydrofiuoric acid charged to the process.

2. A process for concentrating hydrofluoric acid which comprises contacting an aqueous hydrofiuoric acid with a fluorocarbon, separating a solution comprising essentially a fluorocarbon and dissolved hydrofluoric acid from an acqueous solution containing less hydrogen fluoride than the hydroiluoric acid charged to the process, separating the first named solution into hydrofluoric acid and recovered fluorocarbon, recovering said hydrofluoric acid, and recycling the recovered fluorocarbon to contact with the aqueous hydrofluoric acid charged to the process.

3. A process for concentrating hydrofluoric acid which comprises countercurrently contacting an aqueous hydrofluoric acid with a fluorocarbon, separating a solution comprising essentially a uorocarbon and dissolved hydrofluoric acid from an aqueous solution containing less hydrogen iiuoride than the hydrofluoric acid charged to the process, separating the first named solution into hydrofluoric acid and recovered fluorocarbon, recovering said hydrofluoric acid, and recycling the recovered fluorocarbon to contact with the acqueous hydrofluoric acidl charged to the process.

4. A process for concentrating hydrofluoric acid which comprises contacting constant boiling aqueous hydrofluoric acid and a fluorocarbon in liquid phase, separating a solution comprising essentially said fluorocarbon and dissolved hydrofluorio acid from an aqueous solution containing less hydrogen fluoride than constant boiling hydrouoric acid, separating the first named solution into hydrofluoric acid and recovered fluorocarbon, recovering said hydrofluoric acid, and recycling the recovered fluorocarbon to futher contact with the constant boiling aqueous hydrofluoric acid charged to the process.

5. A method for concentrating hydrofluoric acid from its constnat boiling mixture with water which comprises contacting said constant boiling mixture with a liquid fluorocarbon to form an upper less-concentrated hydroiluoric acid layer, and a lower` layer comprising essentially a solution of hydrogen fluoride in said fluorocarbon, separating the layers, and fractionally distilling the lower layer to obtain a distillate comprising essentially hydrofluoric acid of higher hydrogen uoride concentration than constant boiling hydrofluoric acid.

6. A method for concentrating hydrofluoric acid from its constant boiling mixture with water which comprises continuously and countercurrently contacting said constant boiling mixture with a liquid fluorocarbon to form an upper lessconcentrated hydrouoric acid layer and a lower layer comprising essentially a solution of hydrogen fluoride in said fluorocarbon, continuously separating the layers, continuously fractionally distilling the lower layer to obtain a distillate comprising essentially hydrofluoric acid of higher hydrogen fluoride concentration than constant boiling hydrofluoric acid and a residue of recovered iiuorocarbon, and recycling said recovered fluorocarbon to further contact with the constant boiling hydrofluoric acid charged to the process.

` 7. A method for concentrating hydrouoric acid from its constant boiling mixture with water which comprises continuously and countercurrently contacting said constant boiling mixture with a liquid fluorocarbon to form an upper lessconcentratedv hydrofluoric acid layer and a lower layer comprising essentially a solution of hydrogen uoride in said fluorocarbon, continuously separating the layers, continuously fractionally distilling the lower layer to obtain a distillate comprising essentially a hydrouoric acid of from about 90 to about 100% hydrogen fluoride concentration and a residue of recovered fluorocarbon and recycling said recovered luorocarbon to further contact with the constant boiling hydroiiuoric acid charged to the process.

8. A method for concentrating hydrouoric acid from its constant boiling mixture with water which comprises continuously and countercurrently contacting said constant boiling mixture with a liquid peruoroparafn to form an upper less-concentrated hydrofluoric acid layer and a lower layer comprising essentially a solution of hydrogen uoride in said perfluoroparafn, continuously separating the layers, continuously fractionally distilling th-e lower layer to obtain a distillate comprising essentially a hydrofluoric acid of from about 90 to about 100% hydrogen uoride concentration and a residue of recovered perfluoroparalin and recycling said recovered peruoroparaiin to further Contact with the constant boiling hydrofluorie acid charged to the process.

9. A method for concentrating hydrofluoric acid from its constant boiling mixture with water which comprises continuously and countercurrently contacting said constant boiling mixture with a liquid perfluoroheptane to form an upper less-concentrated hydrofluoric acid layer and a lower layer comprising essentially a solution of hydrogen fluoride in said perfluoroheptane, continuously separating the layers, continuously fractionally distilling the lower layer to obtain a distillate comprising essentially a hydrofluoric acid of from about 90 to 100% hydrogen fluoride concentration and a residue of recovered peruoroheptane, and recycling said recovered perfluoroheptane to further contact with the constant boiling hydrofluoric acid charged to the-process.

10. A process for concentrating hydrofluoric acid which comprises contacting an aqueous hydror'luoric acid with a polyluoro-hydrocarbon, separating a solution comprising essentially polynuoro-hydrocarbon and dissolved hydrofluoric acid from an aqueous solution containing less hydrogen uoride than the hydrouoric acid charged to. the process, separating the rst named solution into hydrofluoric acid and polyfluoro-hydrocarbon, and recovering the last-mentioned hydrofluoric acid.

11. A process for concentratingl hydrofluoric acid which comprises contacting an aqueous hydroluoric acid with a uorocarbon, separating a solution comprising essentially a fluorocarbon and dissolved hydroiiuoric acid from an aqueous solution containing less hydrogen iuoride than the hydrofluoric acid charged to the process, separating the rst named solution into hydroluoric acid and uorocarbon, and recovering the lastmentioned hydroluoric acid.

CARL B. LINN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,384,449 Benning et al Sept. 11, 1945 2,388,156 Kelley Oct. 30, 1945 2,423,045 Passino June 24, 1947 2,456,027 Simons Dec. 14, 1948 2,458,551 Benning et al Jan. 11, 1949 2,462,402 Joyce Feb. 22, 1949 2,466,189 Waalkes Apr. 5, 1949 OTHER REFERENCES Fluorine Chemistry, reprint from the Industrial and Eng. Chemistry and Analytical Chemistry, March 1947, pages 239-241, S68-370. 

11. A PROCESS FOR CONCENTRATING HYDROFLUORIC ACID WHICH COMPRISES CONTACTING AN AQUEOUS HYDROFLUORIC ACID WITH A FLUOROCARBON, SEPARATING A SOLUTION COMPRISING ESSENTIALLY A FLUOROCARBON AND DISSOLVED HYDROFLUORIC ACID FROM AN AQUEOUS SOLUTION CONTAINING LESS HYDROGEN FLUORIDE THAN 